It is noted that the waveform distortion of signal light transmitted through an optical fiber occurs owing to the effect of the wavelength dispersion characteristic of the optical fiber that the propagation velocity of the signal light differs depending on a difference in the wavelength thereof. Currently, in an optical transmission system that uses a wavelength division multiplexing (WDM) method the transmission rate of which is greater than or equal to 40 Gbps, a tunable dispersion compensator (TDC) is installed just before an optical receiver corresponding to each wavelength, in order to adjust a wavelength dispersion amount in each wavelength. For example, as described in Japanese Laid-open Patent Publication No. 2002-208892, the bit error rate (BER) of signal light received in an optical receiver is measured during system start-up so that the dispersion compensation amount in the TDC is optimized in response to the wavelength dispersion characteristic of a transmission path in which an optical fiber is used. Using the measurement value as a parameter for the optimization, the dispersion compensation amount in the TDC can be precisely adjusted to an optimum level.
When the above-mentioned optimization of the dispersion compensation amount in which the BER is used as a parameter is also applied during a system operation, the wavelength dispersion characteristic of the transmission path changes owing to environmental conditions such as a temperature change and the like or time degradation. For example, as illustrated in FIG. 1, there is a possibility that a curved line indicating the relationship of the BER to a dispersion compensation amount in a TDC changes from a state indicated by a dashed line to a state indicated by a solid line. In this case, it is difficult to determine whether it is desirable to increase or decrease the dispersion compensation amount in the TDC from the optimum point A1, on the basis of the measurement value B1′ of the BER after the change with respect to the optimum point A1 of a dispersion compensation amount before the change. Namely, when the wavelength dispersion characteristic of the transmission path changes during the system operation, it is difficult to determine the adjustment direction for the optimization of the dispersion compensation amount by simply measuring the BER. As a commonly used technique for avoiding such a situation as described above, there has been a technique in which the dispersion compensation amount in the TDC is changed in both increasing and decreasing directions by requirements, then the BER is measured, and hence the dispersion compensation amount is adjusted in a direction in which the measurement value becomes relatively small, thereby promoting the optimization.
In the optimization of the dispersion compensation amount in the TDC, a wavelength dispersion amount on a transmission path with respect to a signal light having a corresponding wavelength is indirectly obtained on the basis of the BER measured at a receiving end. With respect to techniques for measuring the wavelength dispersion amount on the transmission path, there have been various methods other than the above-mentioned utilization of the BER. For example, there has been disclosed a technique in which a plurality of optical pulses the wavelengths of which are different from one another are transmitted from one end of a transmission path and returned from the other end thereof, the individual optical pulses travelling back and forth on the transmission path are detected, and hence a wavelength dispersion amount is measured on the basis of a corresponding group delay. In addition, there has been disclosed a technique in which a wavelength dispersion amount is measured on the basis of the group delay of an optical pulse caused to travel back and forth more than once on a same transmission path.
Incidentally, when, as described above, the direction of the optimization is determined by increasing and decreasing the dispersion compensation amount in the TDC, the measured BER depends on the quality of a signal light received by an optical receiver. The signal quality is mainly influenced by the level of amplified spontaneous emission (ASE) light occurring when the signal light is optically amplified on the transmission path. The level of the ASE light at a receiving end changes depending on a transmission state such as the transmission distance of the signal light, the number of optical relays (the number of spans), or the like. When the level of the ASE light at the receiving end is high, namely, an optical signal to noise ratio (OSNR) is low, the value of the BER drifts upward at the optimum point of the dispersion compensation amount as illustrated in FIG. 2, in the above-mentioned relationship of the BER to the dispersion compensation amount illustrated in FIG. 1. Accordingly, since the range within which the dispersion compensation amount in the TDC can be adjusted (for example, portions indicated by heavy lines in FIG. 2) narrows, it turns out that a possibility for obtaining an optimum point is reduced. Specifically, when the dispersion compensation amount is increased or decreased in order to determine the direction of the optimization, there is a possibility that the degradation of the signal quality occurs that exceeds a limit within which a forward error correction (FEC) circuit in an optical receiver can correct an error (BER=1*10−3 indicated by a dashed line in the example illustrated in FIG. 2). When such a degradation of the signal quality as described above occurs, it turns out that it is difficult to measure the BER of the signal light, and hence it is difficult to optimize the dispersion compensation amount on the basis of the BER.
In addition, considering a case in which, by applying, in place of the measurement of the BER, a technique of the related art in which the wavelength dispersion amount is measured on the basis of the group delay of the optical pulse caused to travel back and forth on the transmission path, as described above, the optimization of the dispersion compensation amount in the TDC is promoted, a commonly used optical transmission system includes an optical amplifier on a transmission path, and usually an optical isolator is disposed at the input-output portion of the optical amplifier. While the optical isolator allows passage of an optical pulse transmitted from one end of a transmission path and transmitted to the other end thereof, the optical isolator blocks return light reflected from the other end of the transmission path. Therefore, it is difficult to apply the method, in which an optical pulse is caused to travel back and forth on the same transmission path and then the wavelength dispersion amount is measured, to the optimization of the dispersion compensation amount in the TDC in the commonly used optical transmission system. Furthermore, in a case in which it is assumed that an optical transmission system includes no optical amplifier on a transmission path, since an optical pulse travels back and forth on a same transmission path, the power of the optical pulse is reduced owing to the loss of the transmission path. The reduction of the power of the optical pulse becomes noticeable with increase in the number of times the optical pulse travels back and forth on the transmission path. Since the detectable level of the optical pulse is limited, the number of times the optical pulse travels back and forth is restricted. Therefore, if the number of times the optical pulse travels back and forth decreases, it is difficult to measure the wavelength dispersion amount with a required accuracy. Even if the number of times the optical pulse travels back and forth can be increased, the waveform distortion of the optical pulse due to the effect of the wavelength dispersion occurs with increase in the number of times the optical pulse travels back and forth, and hence it is difficult to correctly perform timing detection. Therefore, the measurement accuracy of the wavelength dispersion amount is inevitably reduced.